The objective of the present invention is to provide compositions of dense opal glasses having a "cream white" color which can be used in the manufacture of culinary articles (tableware and ovenware).
As described in U.S. Pat. No. 4,532,223, the purchase of such articles depends, in large measure, upon the aesthetic appearance thereof which is the result, in a general way, of a combination of several elements: the appearance of the opal glass comprising the article; the decoration applied to the material; and the shape of the article. With respect to opal glass, it appears that a "creamy-white" tint similar to that of certain porcelain articles (for example, "Wedgewood Bone China") is particularly desirable.
The objective of this invention is to produce those types of dense, tinted opal glasses. The opalization of those glasses is the result of the precipitation, during the course of cooling the molten glass to a glass body (with or without subsequent heat treatments), of crystal phases of alkali metal and alkaline earth metal fluorides. The principal crystal phase is NaF. One understands that "dense opal glass" means a glass whose opalization, determined through a measure of its opacity, is very considerable.
U.S. Pat. No. 4,532,223 emphasized the difficulty of obtaining a "creamy white" tint in NaF-based opal glasses and provided a new solution consisting of utilizing sulfide ions in the presence of ferric ions as a coloration system. That solution satisfies very well the objective of obtaining the desired tint. However, that system can lead to several difficulties when applied to large scale production. Hence, the tint is sensitive to oxidation/reduction conditions and the glass quality is sometime affected by the presence of gaseous inclusions due to the phenomenon known as "reboil." Whereas they can generally be controlled by observing strict production conditions, those difficulties justify research for a new coloration system for use on an industrial scale.
The goal of the present invention is to provide a system of colorants which would permit, on the one hand, the desired tint to be obtained and, on the other hand, significantly reduce, even eliminate, the previously mentioned difficulties. It must be emphasized that the tint of the opal glass depends not only on the "colorants," but also on the degree of opacity. If this degree of opacity depends upon the cooling rate of the material (in the course of shaping the glass articles, for example, from the molten glass) and any possible heat treatments applied thereto, the degree of opacity is also determined to a critical extent by the nature of the chemical composition of the glass.
It has been proposed to obtain opal glasses by utilizing a combination of Fe.sub.2 O.sub.3 and TiO.sub.2, but the example provided report a large content of CaO and ZnO, thereby raising doubts that they exhibited very great opacity.